This invention relates generally to antenna structures for wireless communications devices, and more particularly to compact, high efficiency, electrically small loop antennas for use in portable communication devices.
The physical size of modem compact communication devices often is dictated by the size of the antenna needed to make them function effectively. To avoid devices that are too large, pagers have made use of electrically small rectangular loop ({fraction (1/10)} wavelength). However, these small antennae tend to be inefficient as a result of their very low radiation resistance and comparatively high resistive loss. Likewise, as a result of their high Q they tend to be sensitive to their physical environment.
To overcome the disadvantages of electrically small loop antennas, there is a continuing need for antennas small in physical dimension; having relatively high efficiency; capable of being placed in close proximity to associated electronic circuits without adversely effecting performance; easy to manufacture using standard, low-cost components; and capable of having radiation patterns altered to support different applications.
This patent application further concerns a circular polarization antenna for left hand and right hand polarization. Circular polarization is typical of satellite systems, such as the Global Positioning System (GPS). This field is in rapid expansion due to the vast range of possible applications and the relative low cost of implementing these systems.
The fixed and mobile land devices associated with such systems have required more specialized antennas designed to perform specific functions effectively. Two types of antennas have to date been used in circular polarization communication and navigation systems on mobile devices: the first is the xe2x80x9chelixxe2x80x9d or helicoidal antenna, while the second is known as the xe2x80x9cpatchxe2x80x9d antenna.
In helicoidal antennas, circular polarization is obtained by exciting a progressive wave on a helicoidal wire; the direction of the circular polarization (left or right) is determined by the sense of helicoidal wire winding.
The helicoidal antennas have the advantage of being very simple to design and produce and have a considerable band width which ensures high sensitivity; this characteristic of the helicoidal antenna makes the tolerance range wider, making it possible to use inexpensive materials which are easy to obtain on the market. This type of antenna has the added advantage of having a good gain value in an axial direction with an equally good axial ratio that, as the experts in the field know, is the most important reference parameter for the quality of circular polarization.
The disadvantage of helicoidal antennas is their by no means negligible height which makes them inconvenient for certain applications, such as installation on vehicles or hand-held devices where low profile antennas are required, obviously because they must be streamlined.
The low profile is the main characteristic of the second type of antenna mentioned above, known as the patch antenna, where circular polarization is obtained by exciting a resonant current distribution on a planar conducting surface. The direction of circular polarization is determined by a precise calculation of the position of the xe2x80x9cpoint of excitationxe2x80x9d of the surface.
This type of antenna, however, requires the use of relatively expensive materials, and, above all great precision during setting up and production due to the small tolerances to respect.
Considering the above state-of-the-art, another type of circular polarization two-way antenna was designed with the aim of offering all the advantages of both of the above antennas, without the disadvantages or application limitations of either.
An omni-directional antenna which includes a conductive loop element supported above a conductive ground plane of a wireless communication device by a conductive leg member. The conductive leg member further defines a feedpoint at which the antenna is operatively coupled to the device""s signal generating circuitry. A dielectric element may optionally be disposed between the loop and ground plane.
The improved antenna displays gain in both the vertical and horizontal orientations. The horizontal gain is due to currents in the loop. The vertical gain (perpendicular to the loop and the ground plane) is due to displacement current fields within the conductive leg member disposed between the loop and the ground plane.
Circular polarization is obtained by exciting a wave along a loop wire. The loop defines a closed path, which need not necessarily be a circular path. An antenna including a rectangularly defined loop is also disclosed herein. Different approaches may be utilized to effect wave polarization (left-hand or right-hand); the first consists in exciting the loop wire at two separate points staggered at an angle of 90 degrees with respect to the center of the loop wire and providing a source in phase quadrature. Alternatively, the loop wire may be excited at only one point by discriminating one of the two polarizations by means of a passive probe, a directional probe or other suitable means.
The operational frequency band of the antenna is largely determined by the outside circumference dimension of the conductive loop. The outer circumference dimension is substantially equivalent to xc2xd of the wavelength of the frequency of response. Thus the system performs similarly to a xc2xd wave slot antenna. Tuning of the antenna can be accomplished by adjusting the feed network. Adjusting the width and location of the conductive leg member will transition the frequency and impedance.
Another feature of embodiments of the present invention is a notch element in the conductive leg member. Changes in the notch height can be used to adjust the antenna match. Further tuning can be accomplished by adjusting the width of the ring. As the ring is made wider, the operational frequency range becomes higher.
One advantage of the invention is that the antenna performance is largely independent of the dimensions of the ground plane. Thus the antenna can be readily adapted to different devices having various ground plane dimensions.